Spatial light modulators can form images by individually controlling light received by each picture element (pixel) of the final image. Typically these modulators comprise arrays of individual elements, each element on the modulator corresponding to an image pixel. The control of the light is achieved by allowing or blocking light from a source to reach the image surface. The amount of time the light is allowed on the surface determines the brightness and color of that pixel. Controlling the switching of the elements of the modulator controls that amount of light.
Spatial light modulators can be reflective or transmissive, micromechanical or crystalline. One type of micro-mechanical device is a digital micro-mirror device (DMD), sometimes referred to as a deformable mirror device. The DMD has an array of hundreds or thousands of tiny tilting mirrors. Light incident on the DMD is selectively reflected or not reflected from each mirror to an image plane, to form images. To permit the mirrors to tilt, each mirror is attached to one or more torsion hinges. The mirrors are spaced by means of air gaps, over underlying control circuitry. The control circuitry provides electrostatic forces, via address electrodes, which cause each mirror to selectively tilt.
For optimal operation of a DMD, each mirror should promptly return to its untilted, or equilibrium, position when desired. For a given sticking force at the landing surface, it is possible to define a hinge restoration force that will free the mirrors from a landed state. However, due to other system considerations, such as the desire to operate the DMD at relatively low voltages, it may not be practical to increase the hinge stiffness to a point where all mirrors will reset automatically upon removal of the address signal.
Crystalline modulators rely upon the birefringent nature of liquid crystal material when a field is applied to that material. In a typical liquid crystal light modulator such as twisted nematic (TN), the liquid crystal molecules stack up in a helix fashion through the depth of the cell. With no electric field applied, the polarization vector of the incoming light is rotated as the light passes through the cell. When an electric field is applied, the molecules change their orientation, reducing the amount of polarization vector rotation by an amount proportional to the field strength. By analyzing the output light from the modulator with a polarizer, the polarization vector rotation is translated into an intensity level which is proportional to the angle between the polarizer axis and the light polarization vector.
These types of modulators turn on rather quickly, but rely upon mechanical restorative forces to return the molecules to their twisted orientation when the electric field is removed. This relaxation process is relatively slow and causes degraded image quality when displaying moving imagery at video rates.
A method of resetting the elements of these modulators quickly is needed that does not further complicate the manufacture or increase the cost.